In screen printing production of objects having multiple colors and complex designs, it is necessary that the workpiece or substrate being printed be moved from station to station so that the successive images applied at each various printing station be maintained in accurate registration with one another. This accurate registration insures that the image actually printed is the image desired. In printing very complex, detailed and colorful images, the allowance for error in positioning of the image on the screen and the positioning of the screen frame in the printing apparatus is small. Especially in high volume production situations, the ability to reduce set up time should result in a significant reduction of the actual cost of producing a printed object.
This invention will be described in connection with, although not limited to, a rotary screen printing apparatus which has a plurality of platens carrying workpieces mounted on a common turntable rotatable in a path beneath a circular array of printing stations. Each printing station contains a printing head and a screen holder frame for holding the screen frame. The screen frame has stretched across it the screen material upon which is an exposed image. In this arrangement, each of the screens in the sequence commonly prints a different image on top of the previously printed image, and moreover, this subsequent image is of a different color and design.
Because images in a printing sequence are printed in a layered fashion to produce a single image, it is necessary that each subsequent image is precisely registered. In the screen printing industry, many different methods and apparatus already exist which after enough time eventually provide registration sufficient to print the desired image. However, as mentioned, these methods consume too much undesirable time which is not cost-effective to the printer. The industry is faced with the demand of printing many different and colorful designs on a short notice. For example, many exhibit shows, trade shows, sporting events and charitable events, just to name a few, offer or sell some printed object for either advertisement or souvenir purposes. The volume and design demands of today require that the printer be able to easily change the printing machine for different print jobs.
Today, the printer desires an apparatus and method in which the printing machine down time for this change is relatively short. In addition to job changes, the screens need to be removed from the printing machine to clean the build-up of dry ink from the screens. This procedure requires that the screen be removed from the machine to be washed. In replacing the screen, the registration procedure has to be repeated and can be a time-consuming process. It is therefore desired that the printer be able to quickly remove and re-register the screen in the printing machine. Thus the printing machine must be equipped with a device whereby the screen frame can be quickly replaced to a registered position.
In a method commonly used, the images are all generally centered on the screens, and then, the screen frames are placed in a screen holder or mounting device. The printing cycle is then started, and a trial and error procedure is used. An object is printed upon, and the results ar analyzed to determine necessary screen adjustments. The screen adjustments are accomplished by moving the screen frames to their predicted corrected position and then tightening the clamps. Equipment often has micro-adjustment devices to allow small vernier displacements to allow shifting of the image a thousandths of an inch. This procedure generally may be repeated several times depending on luck and experience of the printer, thereby increasing the cost because the printing machine is non-operative. Also, the screen frame in many printing machines must be removed to clean the screen, and the above described cost inefficient registration procedure must be repeated when returning the replaced screen frame. As a consequence, this method requires experienced operators or operators with patience to set up the machine for each job. Even with the experienced printers, there is commonly undesirable set up times for these jobs and cleaning tasks.
One current registration procedure involves a micro-adjustment device mounted on the screen frame which is positioned at a zero point mark on the screen holder. The purpose of the micro adjustment device is to provide a common starting point for all the screen frames in their respective screen holders. The starting point is generally indicated as a zero point which is marked on the micro-adjustment device. All the screen frames are placed in their printheads having a mark thereon in the same position on each. This mark on the screen frame is precisely centered on one member of each screen frame. The screen frame is then placed in its screen holder with its mark aligned with the zero position of the micro-adjustment device. Once all the screen frames are placed in the printing machine and having the their mark aligned with the zero point of the micro-adjustment device, the trial and error technique is applied.
However, in using this micro-adjustment system the screen frame does not have to be unsecured from its holder. The micro-adjustment device provides knobs or handles which when turned move the screen frame in the direction desired without having to loosen the screen frame from its holder. Thus, this device and method has improved the set up process, but it does not eliminate the cumulative error because the placement of the screen frames in the printing machine has no relationship to the placement of the image on the screen. The placement of the mark on the screen frame also is very critical in that all the marks must be placed on their screen frame in the same position to insure that the starting point for the micro-adjustment device is the same for each printhead. There further is no relationship between the positioning of the screen frame and the image to the printing surface. The absences of these relationships also further prevent this method from being adequate. Again, the printer must have some experience and patience in setting up this system, which as a result, increases the cost and reduces deserved profits.
Currently, devices are available which provide a relationship between the screen frame and the printing surface. However, these devices and methods do not provide the capability to establish a common relationship between the image placement on the screen, the placement in the printing station of the screen frame and the printing surface. These devices do not prevent cumulative error. The present invention provides a method and apparatus for prohibiting much of the cumulative error associated with registering the image and the screen frame in the printing machine.
These devices, such as disclosed in Jennings U.S. Pat. No. 4,738,909, use a collar with pins extending upward from the printing surface plane which are received by a mounting clamp attached to the screen frame. Generally, these pins are very short and require the screen to mate with the printing surface or its plane. The device here is limited to a few printing machines because of this requirement. Many of the machine applicable to these devices generally appear to be of the manual type, and therefore, will not be advantageous to printers using automatic high speed printing machines.
A general object of the invention is to provide a new and improved registration system for a screen printer apparatus.